What type of signal does HART superimpose on the 4-20 mA signal?

What type of signal does HART superimpose on the 4-20 mA signal?

• A. A direct current (DC) digital signal
• B. An AC DC signal
• C. An AC digital signal
• D. A pulsed DC signal

Answer: (C)

HART sends digital information by riding a high-frequency alternating signal on top of the 4-20 mA loop. The digital data are encoded in that AC waveform using frequency-shift keying, so binary information is conveyed by two tones rather than by changing a DC level. This means the main current still represents the process variable (the 4-20 mA), while the digital channel appears as small, fast AC variations superimposed on it. It’s not pulsed DC, since the data aren’t carried by discrete on/off current steps, and it’s not a plain DC digital signal, because the information travels via an alternating (AC) waveform.

HART sends digital information by riding a high-frequency alternating signal on top of the 4-20 mA loop. The digital data are encoded in that AC waveform using frequency-shift keying, so binary information is conveyed by two tones rather than by changing a DC level. This means the main current still represents the process variable (the 4-20 mA), while the digital channel appears as small, fast AC variations superimposed on it. It’s not pulsed DC, since the data aren’t carried by discrete on/off current steps, and it’s not a plain DC digital signal, because the information travels via an alternating (AC) waveform.